1. Field of the Invention
The present invention relates to a process for refining homopolymers and copolymers of propylene. More particularly, it relates to a process for refining homopolymers or copolymers of propylene, which are produced by polymerization or copolymerization of propylene in liquid propylene (referred to as "bulk polymerization" hereinafter), in an industrially advantageous manner.
2. Description of the Prior Art
Bulk polymerization is well-known and widely applied in industry. In comparison with the polymer slurry produced using conventional suspension (or solvent) polymerization (referred to as "conventional solvent polymerization" hereinafter) in which the polymerization is generally carried out in a liquid saturated hydrocarbon solvent having 5 or more, preferably 5 to 7, carbon atoms, the polymer slurry produced using bulk polymerization is quite advantageous in that separation of the polymer produced from the solvent can be carried out very easily by merely subjecting the slurry to reduced pressure because the unsaturated hydrocarbon monomers (mainly propylene) used as a polymerization solvent have such a high vapor pressure that they are easily vaporized.
In general, however, polymers obtained by this bulk polymerization process have some disadvantages because they contain large amounts of both polymers soluble in boiling n-heptane as a by-product and catalyst residues. Accordingly, such polymers have poorer physical properties, as described hereinafter.
As to polymers soluble in boiling n-heptane, when bulk polymerization is carried out, for example, with a combined catalyst of titanium trichloride and triethylaluminum, the produced polymer contains only 75 to 85% by weight of boiling-n-heptane insoluble polymer. Also, when a combined catalyst of titanium trichloride and diethylaluminum chloride is used, the content of boiling n-heptane insoluble polymer is 85 to 95% by weight.
Such polymers containing large amounts of boiling n-heptane soluble polymers have poorer physical properties, particularly stiffness and tackiness. Therefore, these polymers can be used for molding but they are not suitable for films and fibers requiring relatively good physical properties.
As to the catalyst residues, on the other hand, a reduction in the amount of the catalyst residue in bulk polymerization is inherently possible, since the polymerization has a high polymerization rate and can be operated at a high level of catalyst efficiency as compared with conventional solvent polymerization. However, a very large amount of organo-aluminum compounds must be used in order to maintain high levels of catalyst efficiency and selectivity. For example, when bulk polymerization of propylene is carried out in liquid propylene using highly active titanium trichloride and diethylaluminum chloride, as is disclosed in Japanese Patent Application (OPI) No. 34478/1972, high levels of activity and selectivity can not be maintained unless 0.2 g or more of diethylaluminum chloride per kg of liquid propylene is used. When large amounts of the organo-aluminum compound remain in the polymer as a catalyst residue, the produced polymers have a markedly reduced stability, and are easily degraded by heat, thus resulting in foaming and coloration. Consequently, such polymers can not be used when high quality is desired.
In order to obtain high quality propylene polymers, therefore, the boiling n-heptane soluble polymers and catalyst components remaining as residue must be removed.
Extensive investigations were made to improve the quality of such polymers since for a polymer slurry produced using bulk polymerization, polymers soluble in the slurry, residual catalyst dissolved in the slurry and a part of residual catalyst deposited from the slurry can easily be separated from the slurry in a counter-current washing tower using liquid propylene as a washing solvent. This led to an industrially advantageous process for producing homopolymers and copolymers of propylene (i.e., as described in Japanese Patent Application (OPI) Nos. 79589/1975, 102681/1975, 139886/1976, 3679/1977).
With this process, however, there is the possibility of problems occurring when much higher quality levels than required commercially at present are desired.
For example, in the practical use of the above process, high molecular weight polymers are easily produced in the washing tower. Since film grade polypropylenes account for a large percentage of the polypropylene produced, films obtained from such polypropylenes have many fish eyes due to the high molecular weight polymers, and thus the transparency of the films is too poor for commercial use.